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Newton's First Law: Inertia and Unbalanced Forces

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Newton's First Law: Inertia and Unbalanced Forces Newton’s First Law: Duration: Inertia and Unbalanced Forces 1-2 class periods Essential Questions: About this Poster • What are the proper- ties of inertia? The Swift Gamma-Ray Burst Explorer is a NASA mission which is observ- ing the highest energy explosions in the Universe: gamma-ray bursts • How do common (GRBs). Launched in November, 2004, Swift is detecting and observing experiences with unbal- hundreds of these explosions, vastly increasing scientists’ knowledge of anced forces help us to these enigmatic events. Education and public outreach (E/PO) is one of understand Newton’s the goals of the mission. The NASA E/PO Group at Sonoma State Uni- First Law? versity develops classroom activities inspired by the science and technol- ogy of the Swift mission, which are aligned with the national standards. The front of the poster illustrates Newton’s First Law. Descriptions of the drawings can be found on the next page. This poster and activity are part Objectives: Students will… of a set of four educational wallsheets aimed at grades 6-9; they can be displayed as a set or separately in the classroom. • see that an object at rest remains at rest unless an The activity provides a simple illustration of Newton’s First Law. The unbalanced force acts on it activity is complete and ready to use in your classroom; the only extra • see that an object in motion materials you need are listed on p. 6. The activity is designed and laid out will remain in motion unless so that you can easily make copies of the student worksheet and the other acted upon by an unbalanced handouts. force The NASA E/PO Group at Sonoma State University: • see that an object in motion • Prof. Lynn Cominsky: Project Director will change that motion if • Dr. Phil Plait: Education Resource Director acted upon by an unbalanced • Sarah Silva: Program Manager force • Tim Graves: Information Technology Consultant • Aurore Simonnet: Scientific Illustrator • Laura Dilbeck: Project Assistant We gratefully acknowledge the advice and assistance of Dr. Kevin McLin, Science Concept: the NASA Astrophysics division Educator Ambassador (EA) team, and Newton’s First Law of the WestEd educator review panel. This poster set represents an extensive motion states that a body revision of the materials created in 2000 by Dr. Laura Whitlock and Kara at rest will remain at rest Granger for the Swift E/PO program. unless acted upon by The Swift Education and Public Outreach website is an unbalanced force. It http://swift.sonoma.edu also states that a body in This poster and other Swift educational materials can be found at: motion will maintain that http:// swift.sonoma.edu/education/ motion, in the same direc- National Science Education Standards and Mathematics Standards for tion and with the same the set of four Newton’s Law wallsheets can be found at: speed, unless acted upon http://swift.sonoma.edu/education/newton/standards.html by an unbalanced force. 1 Description of the Front of the Poster: Figure skater: To begin moving, a figure skater must apply a force using her skates. Once in motion, she’ll con- tinue to glide along the ice in a straight line for a long time unless she applies another force. Hands pulling on rope: When each end of a rope is pulled, the rope will move in the direction of whoever is pulling harder – whoever is applying more force. In this case, the magnitude or strength of A (on the right) is greater than that of B (on the left), so the rope accelerates to the right. Snowboarder: A snowboarder experiences a force due to gravity which pulls her down. She will move in a straight line unless she applies a force to the board, changing direction. Train: A train is a very massive object, and therefore has a lot of inertia. Once in motion, it is very difficult to stop, requiring a very large force to slow it. Jogger: A jogger experiences many forces while running: gravity, the push of her feet, the friction of her shoes on the ground, and air resistance. Her legs, together with the friction of her shoes, overcomes her inertia to propel her forward. Car hitting the wall: A car rolling down a hill is being moved by the force of gravity. When the car hits the wall, the greater inertia of the wall stops it. But anything not attached to the car will still move forward, so the man running after the car will lose his coffee, his lunch, and his briefcase. Background Information for Teachers: Sir Isaac Newton (1642-1727) established the scientific laws that govern 99% or more of our everyday experiences – from how the Moon orbits the Earth and the planets orbit the Sun to how a hockey puck slides over ice, a person rides a bicycle, or a rocket launches a satellite into space. Newton’s Laws are considered by many to be the most important laws of all physical sci- ence. They are also a great way to introduce students to the concepts, applications, vocabulary, and methods of science. Sir Isaac Newton Newton’s Laws are related to the concept of motion: Why does an object move the way it does? How does the object accelerate or decelerate? To understand these things, we need to understand the relationship between force and motion. Forces can cause motion. But what exactly is a force? We can think of a force as a push or a pull. A force has a direc- tion as well as a magnitude; such quantities are called vectors. In a diagram, a force can be represented by an arrow indicating its two qualities: The direction of the arrow shows the direction of the force (push or pull). The length of the arrow is proportional to the magnitude (or strength) of the force. Historical Perspective Built upon foundations laid primarily by Aristotle and Galileo, Sir Isaac Newton’s First Law of Motion explains the connection between force and motion. Aristotle theorized that a force is required to keep an object in motion. He believed that the greater the force was on a body, the greater the speed of that body. His theory was widely accepted, since it basically agreed with life’s everyday experiences. Aristotle’s theory remained largely undisputed for almost 2000 years, when Galileo came to a different conclusion. Galileo understood that our everyday experiences had friction in them. He imagined a world without friction, and came to the conclusion that it was just as natural for a body to be in horizontal motion at a constant speed 2 as it was for it to be at rest. It was only in our imperfect, friction-filled world that we needed to continue to push an object to get it to move. Isaac Newton built upon Galileo’s ideas. He agreed that an object would continue to move even if no force acted on it. He also understood that more than one force can act on an object at the same time. The combi- nation of these forces is important. For example, imagine two teams playing tug-of-war; each pulls on a rope in opposite directions. If one team is stronger, then their force is greater and they pull the other team toward them. In this situation, when two forces are not equal, we say they are unbalanced. However, if the two teams have equal strength, the force they apply to the rope is equal – balanced– and neither team moves. In his work known as the “Principia,” published in 1687, Newton wrote about his ideas on forces and motion (and readily acknowledged his debt to Galileo). He created three laws, today called Newton’s Laws of Motion. His First Law of Motion stated: A body continues at rest or in motion in a straight line with a constant speed until acted on by an unbalanced force. The tendency of a body to resist change is called inertia. Newton’s First Law is often referred to as the Law of Inertia. Newton’s Laws apply to macroscopic systems – things you can feel and see. There are environments for which Newton’s Laws (or Classical Mechanics) only provide an approximate answer, and more general physical laws must be used. For example, black holes and objects moving at nearly the speed of light are more accurately explained by General Relativity, while subatomic particles are explained by Quantum Mechanics. Pre-Activity Reading: Newton’s First Law and the Swift Satellite On November 20, 2004, the Swift satellite was sealed in the nosecone of a Delta 2 rocket, ready for launch from Cape Canaveral, Florida. Immediately prior to launch, Swift was “an object at rest” and so was the rocket. There was no unbalanced force on Swift or the rocket, so both of them remained at rest. When the solid rocket boosters ignited at 12:16:00 p.m. EST, an unbalanced force was applied to the rocket. For the first few seconds of the launch, the rocket exhaust went straight down, pushing the rocket straight up, in a line. You can see the Swift launch in a video at: http://www.nasa.gov/mission_pages/swift/multimedia/index.html Pre-Activity Discussion: Ask the following questions to introduce Newton’s First Law to your class: • When were the Swift satellite and rocket at rest? • When were the Swift satellite and rocket in motion in a straight line? • What happens when you are riding in a car with a seat belt on, and the car starts or stops suddenly? • What would happen if you were not wearing your seat belt? • What is providing the unbalanced force for the Swift launch? For the car? • Can you think of some more examples when your body is in motion and it is acted on by an unbal- anced force? 3 Answers to Non-Swift Pre-Activity Discussion Questions: When you are riding in a car with a seat belt on, and the car starts suddenly, you feel the back of the seat push against your back as the car begins to move.
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